# std::ranges::views::single, std::ranges::single_view

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< cpp‎ | ranges

Defined in header `<ranges>` |  |
| (1) |
template< std::copy_constructible T > requires std::is_object_v<T> class single_view : public ranges::view_interface<single_view<T>> |  | (since C++20) (until C++23)
template< std::move_constructible T > requires std::is_object_v<T> class single_view : public ranges::view_interface<single_view<T>> |  | (since C++23)
namespace views { inline constexpr /* unspecified */ single = /* unspecified */; } | (2) | (since C++20)
Call signature |  |
template< class T > requires /* see below */ constexpr /* see below */ single( T&& t ); |  | (since C++20)

1) Produces a `view` that contains exactly one element of a specified value.
2) The expression views::single(e) is expression-equivalent to single_view<std::decay_t<decltype((e))>>(e) for any suitable subexpression e.

The lifetime of the element is bound to the parent `single_view`. Copying `single_view` makes a copy of the element.

### Customization point objects

The name `views::single` denotes a customization point object, which is a const function object of a literal `semiregular` class type. For exposition purposes, the cv-unqualified version of its type is denoted as `__single_fn`.

All instances of `__single_fn` are equal. The effects of invoking different instances of type `__single_fn` on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, `views::single` can be copied freely and its copies can be used interchangeably.

Given a set of types `Args...`, if std::declval<Args>()... meet the requirements for arguments to `views::single` above, `__single_fn` models

- std::invocable<__single_fn, Args...>,
- std::invocable<const __single_fn, Args...>,
- std::invocable<__single_fn&, Args...>, and
- std::invocable<const __single_fn&, Args...>.

Otherwise, no function call operator of `__single_fn` participates in overload resolution.

### Data members

Member | Definition
`copyable-box`﻿`<T>` `value_` (until C++23) | the single element of the view (exposition-only member object*)
`movable-box`﻿`<T>` `value_` (since C++23) | the single element of the view (exposition-only member object*)

### Member functions

(constructor) | constructs a `single_view` (public member function)
begin | returns a pointer to the element (public member function)
end | returns a pointer past the element (public member function)
empty [static] | returns false (public static member function)
size [static] | returns 1 (public static member function)
data | returns a pointer to the element (public member function)
##### Inherited from std::ranges::view_interface
cbegin (C++23) | returns a constant iterator to the beginning of the range. (public member function of `std::ranges::view_interface<D>`)
cend (C++23) | returns a sentinel for the constant iterator of the range. (public member function of `std::ranges::view_interface<D>`)
operator bool | returns whether the derived view is not empty. Provided if ranges::empty is applicable to it. (public member function of `std::ranges::view_interface<D>`)
front | returns the first element in the derived view. Provided if it satisfies `forward_range`. (public member function of `std::ranges::view_interface<D>`)
back | returns the last element in the derived view. Provided if it satisfies `bidirectional_range` and `common_range`. (public member function of `std::ranges::view_interface<D>`)
operator[] | returns the `n`th element in the derived view. Provided if it satisfies `random_access_range`. (public member function of `std::ranges::view_interface<D>`)

## std::ranges::single_view::single_view

single_view() requires std::default_initializable<T> = default; | (1) | (since C++20)
| (2) |
constexpr explicit single_view( const T& t ); |  | (since C++20) (until C++23)
constexpr explicit single_view( const T& t ) requires std::copy_constructible<T>; |  | (since C++23)
constexpr explicit single_view( T&& t ); | (3) | (since C++20)
template< class... Args > requires std::constructible_from<T, Args...> constexpr explicit single_view( std::in_place_t, Args&&... args ); | (4) | (since C++20)

Constructs a `single_view`.

1) Default initializes `value_`, which value-initializes its contained value.
2) Initializes `value_` with t.
3) Initializes `value_` with std::move(t).
4) Initializes `value_` as if by `value_`{std::in_place, std::forward<Args>(args)...}.

## std::ranges::single_view::begin

constexpr T* begin() noexcept; constexpr const T* begin() const noexcept; |  | (since C++20)

Equivalent to return data();.

## std::ranges::single_view::end

constexpr T* end() noexcept; constexpr const T* end() const noexcept; |  | (since C++20)

Equivalent to return data() + 1;.

## std::ranges::single_view::empty

static constexpr bool empty() noexcept; |  | (since C++20)

Equivalent to return false;.

## std::ranges::single_view::size

static constexpr std::size_t size() noexcept; |  | (since C++20)

Equivalent to return 1;.

Makes `single_view` model /*tiny-range*/ as required by `split_view`.

## std::ranges::single_view::data

constexpr T* data() noexcept; constexpr const T* data() const noexcept; |  | (since C++20)

Returns a pointer to the contained value of `value_`. The behavior is undefined if `value_` does not contains a value.

### Deduction guides

template< class T > single_view( T ) -> single_view<T>; |  | (since C++20)

### Notes

For a `single_view`, the inherited `empty` member function always returns false, and the inherited operator bool conversion function always returns true.

### Example

Run this code

    #include <iomanip>
    #include <iostream>
    #include <ranges>
    #include <string>
    #include <tuple>

    int main()
    {
        constexpr std::ranges::single_view sv1{3.1415}; // uses (const T&) constructor
        static_assert(sv1);
        static_assert(not sv1.empty());

        std::cout << "1) *sv1.data(): " << *sv1.data() << '\n'
                  << "2) *sv1.begin(): " << *sv1.begin() << '\n'
                  << "3)  sv1.size(): " << sv1.size() << '\n'
                  << "4)  distance: " << std::distance(sv1.begin(), sv1.end()) << '\n';

        std::string str{"C++20"};
        std::cout << "5)  str = " << std::quoted(str) << '\n';
        std::ranges::single_view sv2{std::move(str)}; // uses (T&&) constructor
        std::cout << "6) *sv2.data(): " << std::quoted(*sv2.data()) << '\n'
                  << "7)  str = " << std::quoted(str) << '\n';

        std::ranges::single_view<std::tuple<int, double, std::string>>
            sv3{std::in_place, 42, 3.14, "😄"}; // uses (std::in_place_t, Args&&... args)

        std::cout << "8)  sv3 holds a tuple: { "
                  << std::get<0>(sv3[0]) << ", "
                  << std::get<1>(sv3[0]) << ", "
                  << std::get<2>(sv3[0]) << " }\n";
    }

Output:

    1) *sv1.data(): 3.1415
    2) *sv1.begin(): 3.1415
    3)  sv1.size(): 1
    4)  distance: 1
    5)  str = "C++20"
    6) *sv2.data(): "C++20"
    7)  str = ""
    8)  sv3 holds a tuple: { 42, 3.14, 😄 }

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3428 | C++20 | `single_view` was convertible from std::in_place_t | the constructor is made explicit
LWG 4035 | C++20 | `single_view` did not provide the member function `empty()` | provides `empty()`
P2367R0 | C++20 | deduction guides for `single_view` failed to decay the argument; `views::single` copied but not wrapped a `single_view` | a decaying guide provided; made always wrapping

### See also

optional (C++17) | a wrapper that may or may not hold an object (class template)
ranges::empty_viewviews::empty (C++20) | an empty `view` with no elements (class template) (variable template)
ranges::split_viewviews::split (C++20) | a `view` over the subranges obtained from splitting another `view` using a delimiter (class template) (range adaptor object)

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